A companion subwoofer for the dipole project

It has been almost one year since my dipole system was completed. One of the shortcomings of the dipoles is that they could not deliver sufficient bass in the 20 Hz to 40 Hz range. I had to reduce via the equalizers the signal in that range to avoid damaging the woofers due to excessive cone amplitude. The latest dipole design is shown in Appendix A. I wanted to listen to organ music and therefore decided to build a subwoofer. After reading about a number of projects in the web I set for a band pass system due to its ability to reach the 20 Hz to 30 Hz range. I made a number of experiments and then built the subwoofer shown in Figure 1. It consists of a KLH Model 17 enclosure (9’’X12’’X23’’) and two sono-tubes mounted in a wooden board. The board is fastened to the KLH speaker with six screws and wind nuts so it is removable for transportation and access to the woofer. The sono-tubes are 8 inches in diameter and 4 feet in length. I removed the original KLH drivers and replaced the woofer with a 10 inch Coustic driver, the tweeter was discarded and its mounting hole was used for the port. Then I added a piece of wood to the KLH cabinet to isolate the air volume corresponding to the sealed chamber from the volume of air in the port. The port is one of the sono-tubes; the other sono-tube forms the sealed enclosure to form the band pass system.

It is a band pass system because its behavior is nicely described by band pass equations. I followed the Small papers and did a derivation of the equations for such a band pass enclosure. These equations were programmed in MathCad. My derivation and the MathCad program are shown in the Appendices B and C. It was a lot of fun to actually measure the peaks and valley of the impedance curve and then set the MathCad program parameters to reproduce the performance. Good insights were obtained in this trial and error process.

Figure 2. Near field frequency response of the band pass enclosure including an inductance and graphical equalizer.

Figure 2 shows the near field frequency response right at the port mouth. To achieve this performance an inductance, a graphical equalizer, and some fiber-glass stuffing inside the KLH cabinet needed to be added. The inductance was to further cut the high frequencies; the equalizer to give the last touch to the response; the fiber-glass to avoid the first resonance of the tube of the port. The effect of the resonance can be seen in the frequency response at about 160 Hz. The sealed chamber making the speaker a band pass system is necessary. I was temped to make the speaker a traditional ported one. However, to reduce harmonic distortion it was necessary to include the sealed chamber. Thus the driver works more symmetrical and harmonic distortion is significantly reduced.

Conclusions

I think the subwoofer plays very well and I hear low tones I did not hear before with the dipoles. Bach’s toccata and fugue in D minor by Peter’s Hurford sounds just sensational. The use of the KLH enclosure and sono-tubes made the project very easy to make.

I should now restate that my entire system really sounds spectacular. On of the results of this project is to realize that to set a stereo system is not necessary a large room, a small room can allow an amazing acoustical experience.

Appendix A

Ballistic woofer (most recent dipole with better drives) equalizer settings

32Hz 0 db (set to 0db to protect the woofer)

64Hz 6

125HZ 3

250Hz 10

500Hz 6

1KHz 6

2KHz 0

4KHz -12

8KHz -12

16KHz -12

Appendix B

MathCad program (click here to download it)

Appendix C